Anticipating Change: Sea-level Rise-Induced effects on Estuary Morphology
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During the mid-Holocene, when sea-level rose rapidly, many estuaries formed. When sea-level rise slowed down during the late Holocene, some of these estuaries filled with sediment, where others remained largely unfilled. Vegetation has an important ecoengineering effect and contributes to estuary infilling. It is unclear, however, how net importing estuaries will respond to the current and projected sea-level rise. To this end, we conducted scaled laboratory experiments in the 20 x 3 m tilting flume the Metronome, which can simulate tidal currents. By asymmetric tilting, a flood dominated, net importing estuary was created, and sediment was supplied to the seaward and landward boundaries. A mud simulant was used and seeds of plant species with eco-engineering behaviour were supplied into the flow periodically. The channel network evolution through time was mapped using a recently developed network extraction tool. This tool can extract Fluvial networks from elevation data in an objective way. It was found that sea-level rise enhances sediment mobility which, in combination with Flood dominance leads to higher sediment fluxes to the upstream part of the experiment than in experiments without sea-level rise. Sea-level rise also allows tidal currents to propagate further upstream which promotes channel formation upstream in the experiment. Most importantly, sea-level rise causes the morphological units in estuaries to shift landward and the upstream part of the system to silt up, given enough sediment is available to fill the created accommodation. In the case of low sediment availability, which is common in systems in which dams and dikes are built, coastal squeeze may prevent transgression and estuarine environments will drown. Two examples, the Scheldt and the Wadden sea, are considered to illustrate what the fate of estuaries could be, considering the current and predicted sea-level rise. It is found that if sea-level rise stays on the low end of the predictions (40 cm/100 yrs), the Scheldt and the Wadden sea area may aggrade to maintain a stable position. In case sea-level rises faster, these systems are likely to transgress but are prone to coastal squeeze because of man-made hard boundaries directly landward of them. It is therefore expected that many of these systems will drown in the centuries to come, unless we change our coastal management strategy and create more space for systems to silt up and keep up with sea-level. The findings of this study can contribute to managing estuaries sustainably in terms of sediment budgets, navigability and ecology.